Ion beams are produced from ions extracted from an ion source. An ion source typically employs an ionization chamber connected to a high voltage power supply. The ionization chamber is associated with a source of ionizing energy, such as an arc discharge, energetic electrons from an electron-emitting cathode, or a radio frequency or microwave antenna, for example. A source of desired ion species is introduced into the ionization chamber as a feed material in gaseous or vaporized form where it is exposed to the ionizing energy. Extraction of resultant ions from the chamber through an extraction aperture is based on the electric charge of the ions. An extraction electrode is situated outside of the ionization chamber, aligned with the extraction aperture, and at a voltage below that of the ionization chamber. The electrode draws the ions out, typically forming an ion beam. Depending upon desired use, the beam of ions may be mass-analyzed for establishing mass and energy purity, accelerated, focused and subjected to scanning forces. The beam is then transported to its point of use, for example into a processing chamber. As the result of the precise energy qualities of the ion beam, its ions may be implanted with high accuracy at desired depth into semiconductor substrates.
The precise qualities of the ion beam can be severely affected by condensation and deposit of the feed material or of its decomposition products on surfaces of the ion beam-producing system, and in particular surfaces that affect ionization, ion extraction and acceleration.
The Ion Implantation Process
The conventional method of introducing a dopant element into a semiconductor wafer is by introduction of a controlled energy ion beam for ion implantation. This introduces desired impurity species into the material of the semiconductor substrate to form doped (or “impurity”) regions at desired depth. The impurity elements are selected to bond with the semiconductor material to create electrical carriers, thus altering the electrical conductivity of the semiconductor material. The electrical carriers can either be electrons (generated by N-type dopants) or “holes” (i.e., the absence of an electron), generated by P-type dopants. The concentration of dopant impurities so introduced determines the electrical conductivity of the doped region. Many such N- and P-type impurity regions must be created to form transistor structures, isolation structures and other such electronic structures, which collectively function as a semiconductor device.
To produce an ion beam for ion implantation, a gas or vapor feed material is selected to contain the desired dopant element. The gas or vapor is introduced into the evacuated high voltage ionization chamber while energy is introduced to ionize it. This creates ions which contain the dopant element (for example, in silicon the elements As, P, and Sb are donors or N-type dopants, while B and In are acceptors or P-type dopants). An accelerating electric field is provided by the extraction electrode to extract and accelerate the typically positively-charged ions out of the ionization chamber, creating the desired ion beam. When high purity is required, the beam is transported through mass analysis to select the species to be implanted, as is known in the art. The ion beam is ultimately transported to a processing chamber for implantation into the semiconductor wafer.
Similar technology is used in the fabrication of flat-panel displays (FPD's) which incorporate on-substrate driver circuitry to operate the thin-film transistors which populate the displays. The substrate in this case is a transparent panel such as glass to which a semiconductor layer has been applied. Ion sources used in the manufacturing of FPD's are typically physically large, to create large-area ion beams of boron, phosphorus and arsenic-containing materials, for example, which are directed into a chamber containing the substrate to be implanted. Most FPD implanters do not mass-analyze the ion beam prior to its reaching the substrate.
Ion Contamination
In general, ion beams of N-type dopants such as P or As should not contain any significant portion of P-type dopant ions, and ion beams of P-type dopants such as B or In should not contain any significant portion of N-type dopant ions. Such a condition is called “cross-contamination” and is undesirable. Cross-contamination can occur when source feed materials accumulate in the ion source, and the source feed material is then changed, for example, when first running elemental phosphorus feed material to generate an N-type P+ beam, and then switching to BF3 gas to generate a P-type BF2+ beam.
A serious contamination effect occurs when feed materials accumulate within the ion source so that they interfere with the successful operation of the source. Such a condition invariably has called for removal of the ion source and the extraction electrode for cleaning or replacement, resulting in an extended “down” time of the entire ion implantation system, and consequent loss of productivity.
Many ion sources used in ion implanters for device wafer manufacturing are “hot” sources, that is, they operate by sustaining an arc discharge and generating a dense plasma; the ionization chamber of such a “hot” source can reach an operating temperature of 800C or higher, in many cases substantially reducing the accumulation of solid deposits. In addition, the use of BF3 in such sources to generate boron-containing ion beams further reduces deposits, since in the generation of a BF3 plasma, copious amounts of fluorine ions are generated; fluorine can etch the walls of the ion source, and in particular, recover deposited boron through the chemical production of gaseous BF3. With other feed materials, however, detrimental deposits have formed in hot ion sources. Examples include antimony (Sb) metal, and solid indium (In), the ions of which are used for doping silicon substrates.
Cold ion sources, for example the RF bucket-type ion source which uses an immersed RF antenna to excite the source plasma (see, for example, Leung et al., U.S. Pat. No. 6,094,012, herein incorporated by reference), are used in applications where either the design of the ion source includes permanent magnets which must be kept below their Curie temperature, or the ion source is designed to use thermally-sensitive feed materials which break down if exposed to hot surfaces, or where both of these conditions exist. Cold ion sources suffer more from the deposition of feed materials than do hot sources. The use of halogenated feed materials for producing dopants may help reduce deposits to some extent, however, in certain cases, non-halogen feed materials such as hydrides are preferred over halogenated compounds. For non-halogen applications, ion source feed materials such as gaseous B2H6, AsH3, and PH3 are used. In some cases, elemental As and P are used, in vaporized form. The use of these gases and vapors in cold ion sources has resulted in significant materials deposition and has required the ion source to be removed and cleaned, sometimes frequently. Cold ion sources which use B2H6 and PH3 are in common use today in FPD implantation tools. These ion sources suffer from cross-contamination (between N- and P-type dopants) and also from particle formation due to the presence of deposits. When transported to the substrate, particles negatively impact yield. Cross-contamination effects have historically forced FPD manufacturers to use dedicated ion implanters, one for N-type ions, and one for P-type ions, which has severely affected cost of ownership.
Borohydrides
Borohydride materials such as B10H14 (decaborane) and B18H22 (octadecaborane) have attracted interest as ion implantation source materials. Under the right conditions, these materials form the ions B10Hx+, B10Hx−, B18Hx+, and B18Hx−. When implanted, these ions enable very shallow, high dose P-type implants for shallow junction formation in CMOS manufacturing. Since these materials are solid at room temperature, they must be vaporized and the vapor introduced into the ion source for ionization. They are low-temperature materials (e.g., decaborane melts at 100C, and has a vapor pressure of approximately 0.2 Torr at room temperature; also, decaborane dissociates above 350C), and hence must be used in a cold ion source. They are fragile molecules which are easily dissociated, for example, in hot plasma sources.
Contamination Issues of Borohydrides
Boron hydrides such as decaborane and octadecaborane present a severe deposition problem when used to produce ion beams, due to their propensity for readily dissociating within the ion source. Use of these materials in Bernas-style arc discharge ion sources and also in electron-impact (“soft”) ionization sources, have confirmed that boron-containing deposits accumulate within the ion sources at a substantial rate. Indeed, up to half of the borohydride vapor introduced into the source may stay in the ion source as dissociated, condensed material. Eventually, depending on the design of the ion source, the buildup of condensed material interferes with the operation of the source and necessitates removal and cleaning of the ion source.
Contamination of the extraction electrode has also been a problem when using these materials. Both direct ion beam strike and condensed vapor can form layers that degrade operation of the ion beam formation optics, since these boron-containing layers appear to be electrically insulating. Once an electrically insulating layer is deposited, it accumulates electrical charge and creates vacuum discharges, or so-called “glitches”, upon breakdown. Such instabilities affect the precision quality of the ion beam and can contribute to the creation of contaminating particles.